Session Initiation Protocol (SIP) is a communications protocol for signaling and controlling multimedia communication sessions. These communication sessions may include internet telephony for voice calls and video calls, as well as instant messaging over internet protocol (IP) networks. In many communications system proxy servers (session management servers) may include multiple IP interfaces. These interfaces may be mixed combinations of Internet Protocol version 4 (IPv4) interfaces and Internet Protocol version 6 (IPv6) interfaces, or dual mode (IPv4/IPv6) interfaces.
IPv4 interfaces require different IP addresses for each interface, while IPv6 allows for the assignment of multiple IP addresses per interface. Dual mode interfaces allow assignment of both IPv4 and IPv6 addresses to the interface. Any interface can be a dual mode interface. When communications traverse both IPv4 and IPv6 interfaces and links, addressing of the communications becomes critical and difficult. Also, it may be difficult to setup and configure communications due to firewalls, routers, and other mechanisms of a network that block communications between particular IP addresses.